When Google soft launched its new Gmail composer last October, it did so in a preview release that gave users the option to test it out andturn it off. Not so anymore, because starting today that resizable, pop-up window is now the default interface for its web Gmail client. The widespread change was apparently spurred by abundant positive feedback from users that found the new layout bolstered multitasking -- a claim we're not entirely sold on. Regardless of your feelings towards this permanent switch, it's headed your way soon and should finish rolling out "over the next few days." So, turn and face the change, people -- it's not like you have a choice.
Source:Official Gmail Blog